Optical velocimetric probe

ABSTRACT

An optical velocimetry probe used in avionics. A measurement volume through which particles can pass is illuminated with a light beam. Light which is backscattered by the particles causes interference with a reference beam which is taken from the original beam. A photodetector detects the interference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the invention is that of optical velocimetric probes fordefining the velocity of particles in relative motion with respect tothe probe.

2. Discussion of the Background

There are currently various optical devices for optically measuring thevelocity of particles having small dimensions (typically less than onemicron).

For this, use may be made of probes operating on the principle of thelongitudinal Doppler effect. A light source sends a beam onto aparticle, and the back-scattered light is analysed; the velocity of thisparticle gives the back-scattered energy a variation in frequency withrespect to the frequency of the incident radiation. In order to obtain asignificant measurement, it is essential to provide a source whosewavelength is perfectly known, in order to determine a precise variationwith respect to the said wavelength.

Other optical systems currently known comprise means for making twoincident rays interfere in a measurement volume through which particleswhose velocity is to be measured pass. The energy back-scattered by theparticles carries an indication of the velocity. The benefit ofinterferometry resides in the fact that it is not necessary to provide aperfectly monochromatic source. By way of example, FIG. 1 illustrates adevice which uses this type of interference.

A source 11 of the diode laser type emits a light beam L₁ in thedirection of an optical splitter 12 which can generate two light beamsL₁₁ and L₁₂ in the direction of a return optical system MR, so that thetwo beams L₁₁ and L₁₂ interfere in the measurement volume MV, by meansof a lens 13, and thereby create interference fringes which are crossedby a particle P passing through the measurement volume in the directionDZ indicated in FIG. 1. The said particle back-scatters light, in theform of the beam L₂, and this light is collimated through the lens 13then focused through a lens 14 in the direction of photodetection means15. The means are coupled to a signal processing device 16 which canextract information relating to the velocity of the particle on thebasis of the electrical signal delivered by the photodetection means 15.

The major problem of this type of conventional fringe probe resides inthe weak energy back-scattered by submicron particles, this energy beingpoorly suited to the sensitivity of conventional photodetectors.

SUMMARY OF THE INVENTION

This is why the invention proposes a velocimetric optical probecomprising means for amplifying the back-scattered energy.

More precisely, the invention relates to an optical velocimetric probecomprising means for illuminating, with a light beam 1, a measurementvolume through which particles can pass which are in relative motionwith respect to the probe, and means for optical detection of a lightbeam 2 comprising a light beam 3 back-scattered by the particles, inorder to produce an electrical signal in response to the passage of aparticle through the measurement volume (representative of the relativevelocity of the particle with respect to the probe), characterized inthat it includes means for producing interference between a referencelight beam 4 derived from the light beam 1 and the light beam 3, thesaid interference being contained in the light beam 2 and the wavefrontsof the light beams 3 and 4 being of the same geometry.

This type of probe is particularly well-suited to aeronauticalapplications, using the velocity of particles constituting atmosphericaerosols.

It may, in particular, satisfy requirements in this field insofar as, atpresent, devices for measuring the velocity of an aircraft compriseprotuberances, arranged at the front of the aircraft and also referredto as Pitot tubes, which determine the total pressure and the staticpressure. Since this type of device is placed outside the aircraft, itneeds to be heated constantly so as not to ice up and so that itprovides reliable information. Furthermore, these are protuberanceswhich entail a non-negligible increase in the aerodynamic drag of theaircraft, which increases consumption.

It should be noted that, in the field of avionics, optical velocimetricprobes based on the aforementioned longitudinal Doppler effect would beunsatisfactory insofar as on-board devices should not comprise sourceswhich are too bulky, but instead sources of the diode laser type, whichhave weaker power and have excessive spectral emission widths ill-suitedto the requirements involved with the longitudinal Doppler effect.

According to a variant of the invention, the optical velocimetric probemay include means for making the wavefronts of the beams 3 and 4identical, and in particular in the case where the delivered light beam1 is parallel and has a plane wavefront, whereas the back-scatteredlight beam 3 is divergent and has a spherical wavefront.

According to a variant of the invention, the optical velocimetric probemay, as means for producing the light beam 1, include a source of thediode laser type delivering an output beam 1 which is divergent and ofelliptical cross-section. This source is advantageously coupled to acollimating/anamorphic assembly capable of producing a parallel lightbeam of circular cross-section.

The velocimetric probe may also include means for deviating the lightbeam 1 so as to bring at least a part of this beam in the direction ofthe light beam 3.

It may include means for focusing the light beams 3 and 4, so as to makethe said beams interfere in the vicinity of the optical detection means.

According to a variant of the invention, the optical velocimetric probecomprises means for determining a plurality of spatial components of thevelocity of the particles with respect to the probe.

More precisely, the velocimetric probe is one which is characterized inthat it includes means for alternately producing at least two lightbeams 1 and 1' illuminating the measurement volume at differentincidences, so as to detect different spatial components of the relativevelocity of the particle with respect to the probe.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be understood more clearly, and other advantages willemerge, on reading the following description which is given withoutimplying any limitation and with the aid of the appended figures, inwhich:

FIG. 1 illustrates an optical velocimetric probe according to the priorart;

FIG. 2 illustrates an illustrative embodiment of an optical probeaccording to the invention;

FIG. 3 illustrates the measurement volume MV₁ irradiated by the lightbeam 1;

FIG. 4 illustrates an example of a velocimetric probe according to theinvention, using a removable mirror so as to create two light beams 1and 1' incident on the measurement volume.

DISCUSSION OF THE PREFERRED EMBODIMENTS

In general, the optical velocimetric probe according to the inventionessentially comprises three parts:

a first part, which may be referred to as the emission channel 1,originating from the means for illuminating a measurement volume with alight beam;

a second part, referred to as the reference channel, constituting thelight beam 4;

a third part which collects the energy back-scattered by particlespassing through the measurement volume and which focuses it in thevicinity of the optical detection means, where this energy interfereswith the reference channel; this last part is referred to as thereception channel.

The various channels, emission, reference and reception, in anillustrative embodiment according to the invention, serve to demonstratethe various characteristics of the invention. The optical velocimetricprobe is one which is designed for aeronautical applications that can beinstalled on board an aircraft, making it possible to estimate thevelocity of the aircraft precisely, by estimating the relative velocityof atmospheric aerosol particles with respect to the said aircraft.

FIG. 2 illustrates the entire on-board probe, in which the first part,referred to as the emission channel, creates a measurement volumeoutside the probe, at a distance which may typically be of the order of100 mm. This emission channel comprises:

a transverse and longitudinal single-mode diode laser 21 which has highpower, more than about 100 mW, and delivers the light beam 1;

an optical assembly composed of a collimator 32 and an anamorphic system33. The collimator makes it possible to convert the divergent light beamdelivered by the diode laser into a parallel light beam. For its part,the anamorphic system makes it possible to change the ellipticalcross-section of the said beam into a circular cross-section having asmall diameter, of the order of one millimeter;

deviation means comprising a prism Pr and a mirror M₁ ;

an optical emission system 01 which focuses the beam 1 at its focus.This creates the measurement volume, the characteristics of which are asfollows.

Distance from the emission lens 01: 100 mm

Diameter at 1/e² distance: 100 μm

Incidence/optical axis: 100 mrad

Geometry of the wavefront: plane

The reference channel comprises:

a part which is in common with the emission channel and is composed ofthe diode laser, the collimator and the anamorphic system;

starting from the prism Pr, the emission channel and the referencechannel split. Almost all of the energy (around 96%) is sent to theemission channel. The remaining part 3 of the energy (about 4%) isreflected to the reference channel. It is important that excessiveenergy which is capable of saturating the optical detection means is notsent in the direction of the said detection means;

the reflected beam 3 is deviated by mirrors M₂, M₃, M₄ and M₅ so as tofold the light beam 4 and equalize the optical paths of the light beams3 and 4 on the reference and reception channels;

afocal means composed of the lenses 03 and 04 make it possible to createa large-dimension plane wave whose geometrical characteristics areidentical to those of the beam 3 after it has passed through the lens01;

a semi-silvered plate SR having a low reflection coefficient (about 4%)makes it possible to superimpose the reference channel and the receptionchannel, i.e. beams 4 and 3;

a focusing lens 02 focuses the reference beam 4 at a short distance fromthe detection means, in this case a photodiode 22. This short distancemay typically be close to 2 mm. There is thus a match between theworking surface of the photodiode and the cross-section of the lightbeam 4.

The reception channel essentially comprises, on the path of theback-scattered light beam 3:

the lens 01 which is in common with the emission channel and makes itpossible to convert the spherical wavefront of the light beam scatteredby the aerosol into a plane wavefront;

a narrow-band optical filter F (of the solar filter type);

the focusing lens 02 and the photodiode 22, which are in common with thereference channel;

on-board means 23 for processing the signal, so as to provideindications regarding the desired velocity.

According to another variant of the invention, means are provided forilluminating a particle, moving with a velocity v with respect to theprobe, with a plurality of light beams having different incidences.

By using at least two beams which are sent one after the other, it ispossible to measure two components of the velocity which are in theplane defined by the two light beams.

FIG. 4 illustrates an example in which the mirror M₁ described above iscoupled to a mirror M'₁, in the vicinity of the lens 01. The mirror M₁is retracted periodically, making it possible for either the beam 1 orthe beam 1' to be generated outside the probe. The first beam isinclined with respect to the optical axis by an angle +θ, and the secondbeam is inclined with respect to the optical axis by an angle -θ.

When a particle passes through the measurement volume created by one ofthe two beams, with a velocity v and an incidence a with respect to theoptical axis, the optical signal which is generated is modulated at oneof the frequencies f₁ or f₂, the values of which are respectively:##EQU1##

On the basis of determining the frequencies f₁ and f₂, it is possible tocalculate v and α.

For values of α equal to π/2-θ and π/2+θ, one of the two frequenciescancel out, which makes the measurement more complicated. In order toovercome this drawback, a third light beam may be created, for exampleon the optical axis, so that at least two frequencies are not-zeroirrespective of the incidence.

In order to measure three components of the velocity, the velocimetricprobe according to the invention comprises means for generating threebeams which do not lie in the same plane and which are sent insuccession.

Thus, when a particle passes through the measurement volume generated byone of the three beams, it will create a signal modulated at a frequencydepending on the inclination of the beam with respect to the opticalaxis and the components of the velocity. Knowing the three frequenciesassociated with the three beams makes it possible to determine the threecomponents of the velocity.

In the following paragraph, we will describe the operation of this typeof optical velocimetric probe.

When particles, and in particular particles constituting an atmosphericaerosol, having dimensions very much smaller than the wavelength of alight ray which irradiates them, pass through the measurement volumeMV₁, these particles scatter a spherical wave whose complex amplitudecan be written:

    A.sub.d =A.sub.e r.e.sup.j2π|Δ.sbsp.o.sup.+sin θ|/λ

with A_(e) : amplitude of the emission wave

r: amplitude back-scatter coefficient of the particle

λ: emitted wavelength

z: position of the particle following the z axis, as described in FIG.2.

FIG. 3 illustrates the beam 1 emitted in the direction of themeasurement volume MV₁ through which particles P pass.

θ: inclination of the emission beam with respect to the optical axis

Δ_(o) : optical path from the source to the measurement volume.

After passing through the lenses 01 and 02, the spherical wave relatingto the light beam 3 will be focused in the focal plane of the lens 02.The amplitude of this wave can be written, ignoring optical transmissionfactors:

    A'.sub.d =A.sub.d e.sup.j2πΔ.sbsp.1.sup./λ

with Δ₁ the optical distance separating the measurement volume and thefocal plane of the lens 02.

This leads to: A'_(d) =A_(e) re^(j2)π[Δ.sbsp.0⁺Δ.sbsp.1^(+z) sin θ]/λ

The reference channel generates a spherical wave relating to the lightbeam 4, the amplitude of which can be written:

    A.sub.r =A.sub.ro e.sup.j2πΔ.sbsp.2.sup./λ

Thus, at the photodiode 22, the waves A'_(d) and A_(r) interfere whilehaving spherical wavefronts; the variable part of the interferencephenomenon is:

    A=2rA.sub.e A.sub.ro cos (2π[(Δ.sub.0 +Δ.sub.1 -Δ.sub.2)+z sin θ]/λ

If the optical paths are balanced between the reference and receptionchannels, then:

    Δ.sub.0 +Δ.sub.1 -Δ.sub.2 =0

and

    A=2πA.sub.e A.sub.ro cos (2πz.sin θ/λ)

If the particle passes through the measurement volume at a velocity v,then it is also possible to write:

with z=vt

A=2r.A_(e).A_(ro).cos (2πv.t.sin θ/λ)

A time-domain signal modulated at the following frequency is thusobtained:

f=v sin θ/λ

With a conventional-fringe probe as described in FIG. 1, in which twoincident beams are made to interfere in the measurement volume throughwhich a particle passes, the amplitude of the variable part of theinterference phenomenon can be written:

    A.sub.F =2r.sup.2 A.sup.2.sub.e.cos (2πvt/λ)

The gain in amplification between the prior art probe and the probe ofthe invention is therefore:

    G=A/A.sub.F =r.A.sub.e.A.sub.ro /r.sup.2 A.sub.e2 =A.sub.ro /rAe=[IR/RI.sub.e ].sup.1/2

If I_(R) is the intensity of the reference beam I_(r) =A² ro

I_(e) is the intensity of the emitted beam I_(e) =Ae²

R is the intensity reflection coefficient of the particle R=r²

In conventional fashion, I_(e) is of the order of 10⁻¹ W

I_(R) is of the order of 10⁻⁴ W

for a value R of the order of 10⁻⁷.

F˜[10⁻⁴ /10⁻¹×10⁻⁷ ]^(1/2) 100

I claim:
 1. An optical velocimetric probe configured to measureparticles in relative motion with respect to the probe, said probecomprising:an illuminating device configured to illuminate, with a firstlight beam, a measurement volume through which said particles pass,wherein said illuminating device includes a focusing means configured tofocus said first light beam on said measurement volume; means foroptical detection of a second light beam which includes a third lightbeam back scattered by the particles in order to produce an electricsignal in response to the passage of one of said particles through themeasurement volume; a device configured to provide a reference lightbeam from said first light beam; a device configured to produceinterference between said reference light beam and said third light beamwherein said interference is contained in said second light beam,wherein a wavefront of the third light beam and a wavefront of thereference light beam are planar and have substantially identicalgeometries when interfering.
 2. The probe according to claim 1 includinga device for deviating a portion of said first light beam in order toprovide said reference light beam.
 3. The probe according to claim 2wherein said device for deviating includes a prism.
 4. The probeaccording to claim 1 further including a device for alternatelyproducing at least two first light beams for illuminating themeasurement volume at different in order to detect different spatialcomponents of the relative velocity with a particle with respect to theprobe.
 5. The probe according to claim 4 wherein said device forproducing at least two light beams includes a single illumination sourceand at least one removable mirror in a path of said at least two lightbeams.
 6. The probe according to claim 2 further including a device foralternately producing at least two first light beams for illuminatingthe measurement volume at different angles of incidence in order todetect different spatial components of the relative velocity with aparticle with respect to the probe.
 7. The probe according to claim 3further including a device for alternately producing at least two firstlight beams for illuminating the measurement volume at different anglesof incidence in order to detect different spatial components of therelative velocity of a particle with respect to the probe.
 8. An opticalvelocimetric probe configured to measure particles in relative motionwith respect to the probe, said probe comprising:an illuminating deviceconfigured to alternately produce at least two first light beams whichilluminate a measurement volume at different angles of incidence,wherein said illuminating device includes a focusing means configured tofocus said first light beams on said measurement volume and saidparticles pass through said measurement volume; means for opticaldetection of a second light beam which includes alternate third lightbeams, corresponding to said first light beams, back scattered by theparticles in order to produce an electric signal in response to thepassage of one of said particles through the measurement volume; adevice configured to provide a reference light beam as a plane wave fromat least one of said first light beams; a device configured to produceinterference between said reference light beam and said third lightbeams wherein said interference is contained in said second light beam;a processor adapted to derive different spatial components of a relativevelocity of a particle with respect to the probe based on the detectionof said second light beam.
 9. The probe according to claim 8 furtherincluding a device configured to form said third light beams as planewaves in order to produce interference with said plane wave referencelight beam.
 10. The probe according to claim 8 including a deviceconfigured to deviate a portion of at least one of said first lightbeams in order to provide said reference light beam.
 11. The probeaccording to claim 10 wherein said device for deviating includes aprism.
 12. The device according to claim 8 wherein said deviceconfigured to produce at least two first light beams includes a singleillumination source and at least one removable mirror in a path of saidat least two light beams.